Here, a typical example of the image bearing member on which the electrostatic latent image is formed is an electrophotographic photosensitive member or a dielectric member for electrostatic recording. As for the image forming apparatus, examples include an electrophotographic type or electrostatic recording type copying machine, printer, facsimile machine or a complex machine thereof, and an image display device or the like.
The description will be made, as an example, to a transfer type electrophotographic image forming apparatus. Generally, in such an apparatus, the electrostatic latent image of image information is formed by charging means for uniformly charging a surface of the image bearing member (rotatable drum type electrophotographic photosensitive member) to a predetermined polarity and potential and by exposure means for selectively exposing the thus charged drum surface to the light of the image information. The latent image is visualized (developed) into a toner image using a developer (toner) by developing means. The toner image is transferred onto a recording material by transferring means. The toner image on the recording material is fixed by a fixing means into a fixed image. The recording material is outputted as a print.
A recently dominant charging means (charging device) is a contact-charging type means using a fixed type charging member such as a blade or film, or a rotating type charging member such as a brush, roller, belt of semiconductive rubber or resin material.
The contact-charging type does not necessitate an ozone removing filter because the amount of produced ozone is small. An applied voltage required to charge the drum surface up to a predetermined potential can be reduced. It is, therefore, advantageous in the downsizing and the low cost.
A charging mechanism of the contact-charging type will be described. It is known that the charging mechanism for the drum surface in the contact charging system is ruled by Paschen's Law relating to the electric discharge in a small gap.
1) In the case of charging roller:
Referring to FIG. 14, parts (a) and (b) are a schematic perspective view and a schematic sectional view of a charging roller using a rotating type charging roller 21 as the charging member. The charging roller 21 comprises an electroconductive core metal and the electroconductive elastic layer 21b formed on the core metal 21a concentrically therewith. The drum 1 comprises an electroconductive drum base member (bare tube) 12 and a photosensitive layer 11 on the outer surface of the drum base member 12. The charging roller 21 is substantially parallel with the drum 1 and is contacted at a predetermined urging force.
The charging roller 21 has a length to cover an image forming region (maximum image region width) G of the surface of the drum 1 and is rotated by the rotation of the drum 1. To the core metal 21a of the charging roller 21, a predetermined charging bias voltage is applied from the charging bias voltage applying source V so that a bias voltage is applied to the elastic layer 21b through the core metal 21a. By this, the surface of the rotating drum 1 is charged uniformly to the predetermined polarity and potential.
Part (c) of FIG. 14 shows an electrical equivalent circuit of the drum 1 and the air layer of the fine gap concerned with the discharge between the charging roller 21 and the drum 1. An impedance of the charging roller 21 is small as compared with that of the drum 1 and that of the air layer, and therefore, it is neglected here. Then, a charging mechanism can be simply expressed by two capacitors C1 and C2. When a DC voltage is applied to the equivalent circuit, it is divided proportionally to the impedances of the capacitors, and therefore, the voltage Vair across the air layer is,Vair=C2/(C1+C2)  (1)
The air layer has a dielectric breakdown voltage determined by Paschen's Law, and it is as follows when the thickness of the air layer is g [micron]:312+6.2 g[V]  (2)
When Vair exceeds this, the discharge occurs.
The minimum discharging voltage is that when formula (1) is equal to formula (2) and the air layer thickness g obtained by the equation has a double root (C1 is also a function of g), and a DC voltage value at this time is a discharge starting voltage Vth. The theoretical value Vth thus obtained is very close to an experiment value.
The charging roller tends to be complicated in the structure since it requires a rotatable supporting member 211, an urging spring 212 and so on for the charging roller 21. A brush charging member (charging brush) is time-consuming in manufacturing the brush, irrespective of whether it is rotating type or fixing type, and tracks of the brush fibers may result in unevenness of charging.
2) In the case of charging blade:
FIG. 15 is a schematic perspective view of a charging blade using a fixed type charging blade 22 as the charging member. The charging blade comprises an electroconductive elastic blade portion 220 as a charging blade 22 and an electroconductive supporting member 223 supporting the blade portion 220. The blade portion 220 has a length enough to cover the entire width of the image forming region of the surface of the drum 1. The charging blade 22 is set substantially parallel with the drum 1, the blade portion 220 is contacted to the drum 1, and the supporting member 223 is fixed to a stationary member.
A predetermined charging bias voltage is applied from a charging bias voltage applying source V to the supporting member 223, so that the bias voltage is applied to the blade portion 220 through the supporting member 223. By this, the surface of the rotating drum 1 is charged uniformly to the predetermined polarity and potential. The discharge occurs in the wedged small gap formed between the blade portion 220 and the drum 1, and a relatively stable small gap can be formed. The rotation supporting member 211 and the urging spring 212 and so on required by the charging roller are unnecessary, and therefore, the blade type is inexpensive.
A scraping amount of the drum 1 in the case of the contact charging type is larger in the charging region than outside thereof, that is, the surface of the drum 1 tends to be scraped. In addition, the scraping of the surface of the drum 1 at an end of the charging region is particularly remarkable. The reasons why the drum 1 scraping is large in the end of the charging region are as follows.
1) at a longitudinal end of the charging member, concentrated discharge occurs from an edge, and therefore, the discharge at the end is more intense than in the longitudinal central portion.
2) as a result of the flexure caused by the contact between the drum and the charging member, the contact pressures therebetween are larger at the opposite longitudinal ends than in the central portion.
For these reasons, the scraping of the drum 1 is remarkable at the longitudinal end positions of the charging member. Because of the recent demand for the downsizing of the device, the longitudinal dimensions of the drum and the charging roller are very close to the width of the image forming region (maximum image forming region width) G, which results in an image defect caused by remarkable scraping of the drum surface at the longitudinal end position of the charging member. Taking into account the fact that the area of the drum surface contacting the cleaning blade also tends to be scraped, it has been proposed that the end positions of the charging roller be located outside the contact area of the cleaning blade (Japanese Laid-open Patent Application 2006-208550).